In general, a chassis made of stainless steel having high mechanical strength is employed in a portable device or the like in order to protect a display portion for displaying an image from an external shock. In Japanese Patent Laying-Open No. 2006-113589, for example, there is disclosed a display device including a panel unit and a chassis for fixing and supporting the panel unit. Although not clearly described in Japanese Patent Laying-Open No. 2006-113589, the chassis of the display device is conceivably made of stainless steel having a thickness of about 0.3 mm to about 0.4 mm, which is the thickness of a typical chassis.
In recent years, on the other hand, both high performance and downsizing (thinning) are desired in the portable device. In the case where the performance of the portable device is improved, an increase in the processing speed of an integrated circuit (IC) such as a CPU (Central Processing Unit) and a corresponding increase in power consumption result in an increase in the amount of heat generation in an electronic component of the portable device. Thus, the chassis of the portable device is required to serve as a heat radiation member in order to downsize the portable device while sufficiently ensuring heat radiation performance.
Furthermore, in Japanese Patent Laying-Open No. 2012-7090, for example, it is disclosed that a heat-conductive pressure-sensitive adhesive sheet whose heat conductivity is improved by containing alumina is provided between an electronic component and a chassis in order to improve heat conductivity between the electronic component and the chassis.
In the chassis made of stainless steel described in Japanese Patent Laying-Open No. 2006-113589, however, the heat conductivity of the stainless steel is relatively low, and hence there has been such an inconvenience that the heat radiation performance of the chassis is insufficient even in the case where the heat-conductive pressure-sensitive adhesive sheet described in Japanese Patent Laying-Open No. 2012-7090 is employed.
The heat radiation performance of the chassis made of stainless steel described in Japanese Patent Laying-Open No. 2006-113589 can be improved by bonding a graphite sheet described in Japanese Patent Laying-Open No. 2010-215441, for example, Cu foil less expensive than the graphite sheet, or the like onto the chassis. However, there has been such an inconvenience that the number of components is increased by adding the graphite sheet or the like and the thickness of the portable device is increased, and the improvement of the heat radiation performance of the chassis has been desired.
Therefore, as a structure for improving the heat radiation performance of the chassis, a heat radiation substrate (chassis) employing stainless steel having high mechanical strength and a Cu alloy having high heat conductivity has been proposed in Japanese Patent Laying-Open No. 2007-12928, for example.
In Japanese Patent Laying-Open No. 2007-12928, there is disclosed a heat radiation substrate 602 including a rectangular fitting component 622 made of a Cu—Mo composite material and a base material 621 made of SUS 304, formed with a rectangular hole 621a into which the fitting component 622 is pressed, as shown in FIG. 14. The fitting component 622 of this heat radiation substrate 602 is formed such that the upper surface 622a thereof is smaller than the lower surface 622b thereof and is formed with steps 622c on the outer surface. The hole 621a of the base material 621 into which the fitting component 622 is pressed is formed to correspond to the shape of the fitting component 622 and is formed with steps 621b on the inner surface of the hole 621a. The fitting component 622 is inserted into the hole 621a of the base material 621 from below (Z2 side) to fit together, whereby the fitting component 622 and the base material 621 are joined to each other.